Lawyers for accused Aurora movie theater shooter James Holmes said he is mentally ill, justifying a plea of not guilty by reason of insanity. The judge said he would decide this month whether to accept the plea change.

Accused movie theater gunman, James Holmes (2nd from r.) is shown in this courtroom artist sketch listening to his legal team during hearing in Centennial, Colo., Monday. A Colorado judge found 'good cause' to allow Holmes, who could face the death penalty if convicted of murdering 12 moviegoers last year, to change his plea to not guilty by reason of insanity.

James Holmes, accused of killing 12 people at an Aurora, Colo., movie theater, sought Monday to change his plea from not guilty to not guilty by reason of insanity. A judge said there was “good cause” to allow the change but that he would not decide whether to accept the new plea until later in the month.

“Allowing Mr. Holmes to enter a plea of not guilty by reason of insanity would be consistent with fairness and justice," Judge Carlos Samour announced to the packed courtroom.

In Colorado, a defendant must enter a plea at their arraignment, but may change it later if they can show strong reasoning for the switch. Mr. Holmes’ counsel said Monday that the change is warranted since they have now had time to consult with experts and obtain a diagnosis of mental illness for their client.

If granted in a hearing on May 31, the new plea sets the stage for the defense to argue that Mr. Holmes could not distinguish between right and wrong when he opened fire at a midnight showing at the suburban movie theater last July. Dozens of people were wounded in that attack.

“Sometimes evil looks a whole lot like insanity,” says Rashmi Goel, a criminal law expert at the University of Denver’s Sturm College of Law. “In general when people commit these kinds of atrocities our human response is to say they must be insane because what sane person would do this? But the standards for medical and legal insanity are not the same.”

Neither is the legal definition of insanity the same in every state. In Colorado, for instance, the burden of proof is on the prosecution to determine “beyond a reasonable doubt” that the defendant is sane once an insanity plea is introduced. In most states, it’s the opposite – the defense must affirmatively prove insanity. (Four states – Kansas, Montana, Idaho and Utah – don’t allow insanity defenses at all.)

In Colorado, however, they must simply show it’s more likely than not that Holmes was insane at the time he walked into a crowded movie theater and opened fire. At the same time, however, the state has a very narrow definition of insanity, which means Holmes’ attorneys will also have a high bar to clear in making their case.

“The reason this tension exists is because we want to serve real justice,” Ms. Goel says. “We want to make sure that assertions of insanity are rare, but also that we don’t punish someone who truly did not know what they were doing.”

To show that Holmes is not guilty by reason of insanity, his counsel would look for proof that he both suffers from a mental illness and did not know what he was doing was wrong at the time he committed the crime.

It is the combination of the two that’s crucial, says Karen Steinhauser, a former prosecutor and Colorado defense attorney, because “people can be severely mentally ill but it doesn’t necessarily mean they didn’t know the morality of their choices.”

Assuming the insanity defense proceeds, a forensic psychiatrist appointed by the state will evaluate Holmes, attempting to unravel the workings of his mind on that July night. For that, the doctor will rely on an intensive evaluation of Holmes himself, as well as conversations with witnesses and close analysis of all of his medical records.

The idea, Ms. Steinhauser says, is to prove as definitively as possible that Holmes was not in a state of mind to properly understand what he was doing.

That’s because in pursuing an insanity defense, Holmes would wave any doctor-patient privilege pertaining to his meeting with the state-appointed psychiatrist. The defense can ask for an outside evaluation as well, but they cannot refuse the state’s analysis.

Judge Samour said Monday he would not formally accept the new plea until he was convinced Holmes understood those consequences.

Holmes was scheduled to go to trial in February 2014, but an insanity defense plea would likely stretch that timeline considerably, giving both the prosecution and defense time to build their case for what happened – and why – on that summer night last year.